This invention relates to the placement of optical fiber cable and coaxial cable in conduit and more particularly to an improved method and apparatus of pulling optical fiber cable into an underground conduit using standard telephone line trucks that do not have to be dedicated to the activity.
With the recognition that optical fiber cables (OFC) would eventually be accepted as a major transmission media for telecommunication systems, cable manufacturers have developed economical high grade optical fiber with an attenuation characteristic (dB of loss per kilometer of length) that is now almost as low as the loss at a splice in an optical fiber. This means that each splice in an optical fiber reduces the overall distance between repeaters by about one kilometer. Optical splices themselves are also costly in terms of labor, equipment and materials. Additionally, each pull of an OFC has fixed set-up charges that are generally unrelated to the length of the cable that is to be pulled. It is desirable therefore to minimize expenses related to and the number of splices in an optical fiber transmission path by pulling longer lengths of cable into cable ducts.
The placement of OFC requires different methods and equipment than are used for placing twisted pair copper conductor cables. This is primarily due to the relatively low value of the maximum pulling tension (600 pounds) prescribed for most OFCs as opposed to an allowable pulling tension of 6,000 pounds for a 1,500 pair copper cable, for example. This limits the maximum length of OFC that can be pulled when the pulling tension in the cable increases with the length of cable that is to be pulled. Additionally, the minimum bending radius for most OFCs is 20 times the cable diameter which is approximately 3/4 inch. It is also desirable therefore to pull OFCs rapidly but with as low a pulling tension as is practical and around corners with a large bending radius.
Various techniques for pulling OFC are described in open literature such as "Development and Installation of an Optical-Fiber Cable for Communications" by J. A. Olszewski, et al, IEEE Transactions on Communications, Volume 26, No. 7, July 1978, pages 991-998; "Installation and Performance of Chicago Lightwave Transmission System" by T. C. Cannon et al, IEEE Transactions on Communications, Volume 26, No. 7, July 1978, pages 1056-1060; "COS 2 Experiment in Turin: Field Test on an Optical Cable in Ducts" by G. Cocito, et al, IEEE Transactions on Communications, Volume 26, No. 7, July 1978, pages 1028-1035; "Optical Transmission for Interoffice Trunks" by E. E. Basch, et at, IEEE Transactions on Communications, Volume 26, No. 7, July 1978, pages 1007-1014; and "Dual function tape cuts working time on Northeast Corridor FO project" by S. T. O'Meara, Telephony, May 30, 1983, pages 59 and 62.
As stated in the literature, flexible one inch inner diameter polyethelene pipes or subducts are most often pulled into four inch diameter duct pipes in the ground for housing OFCs. A liquid lubricant (such as liquid vaseline or Hydralube Blue that is manufactured by Arnco Inc., of Youngstown, Ohio) is then inputed into the subduct prior to and/or during the pulling of an OFC for reducing friction. Approximately 9 gallons of Hydralube Blue is normally used for each kilometer of OFC that is pulled in subducts. The use of Hydralube Blue is currently recommended by cable manufacturers for pulling OFCs and is described in the one page article "Polymer Compound Solves Conduit And Duct Lubricating Problems" in the October 1978 issue of Contractors Electrical Equipment and in U.S. Pat. Nos. 4,111,820; 4,170,673; and 4,181,137, all having a common inventor, Allen C. Conti. In a single pull (i.e., from a single pulling point) made by the inventors here with one manufacturer's OFC, only 3,000 feet of cable could be mechanically pulled with a lightweight pulling line before reaching the 600 pound maximum allowable pulling tension, with the average tension being 450 pounds. During this pull approximately 25 gallons of this lubricant was poured ahead of and/or sprayed onto the cable, with some lubricant being introduced into the subduct at the midpoint of the path which was relatively straight except for one small dip and an associated rise near the midpoint. In single pulls of another manufacturer's OFC with the same type lubricant and lighweight pull line, only 2,000 feet of OFC could be pulled before reaching the 600 pound maximum pulling tension over a long time interval of 8 hours and a path that was also relatively straight and contained only minor corner variations. In another case, less than 1,000 feet of the other manufacturer's OFC could be pulled in a long period of time (i.e., 6 hours and over a relatively straight path, except for one each 4 foot radius 90 degree corner, before reaching the 600 pound maximum allowed pulling tension.
In many instances the pulling mechanism includes a stainless steel pulling cable which is heavy, stretchable and abrasive. Since the pulling tension at the winch will be high and variable due to the weight and stretching of the stainless steel cable, the pulling tension on the OFC must be measured at the pulled end of the cable. This is relatively complex and expensive. Also, such a pulling cable is undesirable since it may scratch or even burn holes in the inner diameter of the subduct, thereby increasing frictional forces that impede movement of the OFC in it. Alternatively, polypropylene rope and aramid fiber tape are also described as a pulling line.
Although the desirability of pulling long lengths of OFC is widely acknowledged, single pulls of OFCs are normally running less than 2,000 feet. Although the pulling of continuous lengths of OFC of as much as 5,000 feet are reported, all of those known to the inventors here require intermediate assistance in the pull and are done with pulling tensions approaching the maximum allowable value thereof. Stated differently, this can mean stationing a human operator in each manhole and having all of the operators simultaneously pull on the cable so that only a short length of cable is being independently pulled from any pulling location. This was the case for the previously reported longest pull of OFC of 5,000 feet in Hawaii which required 38 people. Also, this is one of the ways of pulling OFC that is recommended by cable manufactures, where a crew of 10 people is suggested for pulling up to 1,500 feet of cable through ducts and manholes. For pulls of greater than 1,500 feet, it is recommended that excess cable be pulled out of the 1,500 foot manhole and laid on the ground. The excess cable is then reintroduced into the duct at the 1,500 foot manhole (after the pulling crew has moved up to new manholes) and the pull process repeated for the next 1,500 feet. This requires considerable time in man hours of labor and it is unknown what bending radius or tension is put on the cable by individuals doing the pulling. Alternatively, portable mechanical puller units can be located in manholes at 1,500-2,000 foot intervals where the pull line and/or OFC makes a couple of turns around a large diameter capstan before going on into the next length of duct. All of these units operate simultaneously for pulling a long-continuous length of OFC by means of a series of pulls of short (e.g., 1,500 foot) lengths of cable. This latter method requires 9 or 10 human operators and is therefore costly in terms of both capital equipment and the number of manhours involved in a pull. Additionally, such equipment is generally heavy and bulky so as to require a number of people to move it around, and other equipment requires a vehicle(s) dedicated to the pulling of such cable.
An object of this invention is the provision of improved apparatus for pulling continuous long lengths of OFC.
Another object is the provision of improved apparatus for pulling OFC that may be practiced with a standard utility line truck that does not have to be dedicated to such an activity.